Is |x| = y-z?

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ash_maverick: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Sun Jan 07, 2007 11:13 pm

by ash_maverick » Fri Jun 05, 2009 4:09 am

Is |x| = y-z?

1. x+y=z
2. x<0

I am little poor in cracking the mod problems. I request somebody to enlighten me on mod along with solution to this problem. ThnX

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Source: Beat The GMAT — Data Sufficiency | Fri Jun 05, 2009 4:09 am

mike22629: Master | Next Rank: 500 Posts; Posts: 322; Joined: Fri Mar 27, 2009 3:56 pm; Thanked: 24 times; GMAT Score:710

by mike22629 » Fri Jun 05, 2009 4:46 am

IMO C

First Statement:

x + y = z
x = z - y

Ins.

Second Stmt.

x < 0

Ins.

Together

Sufficient because z - y is opposite of y - z

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mike22629: Master | Next Rank: 500 Posts; Posts: 322; Joined: Fri Mar 27, 2009 3:56 pm; Thanked: 24 times; GMAT Score:710

by mike22629 » Fri Jun 05, 2009 4:48 am

To further elaborate:

absolute value must be positive, but we do not know if y - z is pos or if z - y is pos. Since x is negative, we know that y - z is pos

Hence the statement is true

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Domnu: Master | Next Rank: 500 Posts; Posts: 385; Joined: Sun May 24, 2009 3:55 pm; Thanked: 11 times; GMAT Score:740

by Domnu » Sat Jun 06, 2009 2:25 pm

The easiest way to crack mod questions is by doing the following:

|x| = x (only if x > 0)
|x| = -x (only if x < 0)

Here, it's pretty clear that 2 all by itself doesn't do much. However, if you have that |x| = y - z and x < 0, then -x = y - z, so z = x + y, and you get 1.

Have you wondered how you could have found such a treasure? -T

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ash_maverick: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Sun Jan 07, 2007 11:13 pm

by ash_maverick » Sat Jun 06, 2009 9:36 pm

Domnu wrote:The easiest way to crack mod questions is by doing the following:

|x| = x (only if x > 0)
|x| = -x (only if x < 0)

Here, it's pretty clear that 2 all by itself doesn't do much. However, if you have that |x| = y - z and x < 0, then -x = y - z, so z = x + y, and you get 1.

Domu, isn't it true that what ever be the sign of x [-ve or +ve] the mod of x will always be positive? I mean, lets say, x= -4 so |x|=> |-4|= 4. Could you please throw some more light on it? Got confuse after seeing this line

|x| = -x (only if x < 0)

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Domnu: Master | Next Rank: 500 Posts; Posts: 385; Joined: Sun May 24, 2009 3:55 pm; Thanked: 11 times; GMAT Score:740

by Domnu » Sun Jun 07, 2009 7:54 am

Yes, what you say is true.. |x| > 0. But! Look at this:

|x| = -x

This doesn't mean that there are no solutions.. what if the right side is positive? That is, what if

-x > 0

which is the same thing as writing x < 0? Try putting in a few values:

|-2| = -(-2) = 2 (correct!)

|-pi| = -(-pi) = pi (correct!)

etc.

Generally, the definition of mod is the following:

|x| = x only if x is 0 or positive
|x| = -x only if x is 0 or negative

Have you wondered how you could have found such a treasure? -T

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